1. Field of the Invention
The invention relates to an excimer or molecular fluorine laser, particularly having a resonator configured with adaptive or phase conjugative optics for correcting wavefront distortions generated as a result of thermally-induced refractive index fluctuations of resonator optics.
2. Description of the Related Art
Excimer lasers and molecular fluorine lasers are used for photolithographic production of integrated circuit device. Achromatic imaging optics for the output emission wavelengths of these lasers are difficult to produce. For this reason, line narrowing optical resonators are used for these photolithographic applications to prevent errors caused by chromatic aberrations. Typically acceptable bandwidths for different imaging systems are tabulated in Table 1, below, for the KrF excimer laser emitting at around 248 nm, the ArF excimer laser emitting at around 193 nm and the molecular fluorine laser emitting at around 157 nm.
Current lithography lasers operate at repetition rates typically of up to 2 kHz. To produce higher throughput, it is desired to operate these lithography lasers at higher repetition rates such as 4 kHz or more (e.g., even 10 kHz or more). The averaged power incident upon optical elements within the laser cavity generally increases as the laser is operated at higher and higher repetition rates, and will rise by a factor of two or more when the repetition rate is increased from 2 kHz to 4 kHz or more. A very high thermal load on intracavity optical components, especially of narrow band optics such as prisms or etalons, can cause undesirable wavefront distortions in the laser beam, even at 1-2 kHz, and especially at higher repetition rates. These wavefront distortions are caused by thermally induced changes of the refractive indices of materials of the intracavity optical components resulting in time dependent variations of the spectral distribution of the laser beam, and of near and far field intensity distributions. It is desired to have a high power laser, particularly for photolithographic applications, wherein effects of wavefront distortions are reduced or prevented, such as by providing a resonator having intracavity wavefront correction or compensation.
In view of the above, a gas discharge laser system is provided including a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, and a resonator including the discharge chamber therein for generating a laser beam. The resonator further includes a retro-reflective array for making wavefront corrections to the laser beam, a beam expander before the retro-reflective array for increasing a radius of curvature of the wavefront associated with the laser beam incident on the beam expander, and one or more line-narrowing optical elements for narrowing a bandwidth of the laser beam.
The retro-reflective array may have a non-planar contour fit approximately to an average wavefront contour of the laser beam incident upon the array for making the wavefront corrections. The non-planar contour may be adjustable for controlling the contour to fit the wavefront contour. In this case, the laser system may include a detection and control system including a detector and a processor for monitoring a parameter of the laser beam indicative of the fitting of the contour of the retro-reflective array to the contour of the wavefront. The processor may be connected in a feedback loop with the array for controlling the contour of the array based upon information received from the detector. The detection and control system may also include a spectrometer.
In further view of the above, a gas discharge laser system is provided including a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, and a resonator including the discharge chamber therein for generating a laser beam. The resonator further includes a phase conjugate mirror for making wavefront corrections, and one or more line-narrowing optical elements for narrowing a bandwidth of the laser beam. The resonator may further include a beam expander before the phase conjugate mirror for increasing a radius of curvature of a wavefront of the laser beam incident on the phase conjugate mirror.
In further view of the above, a gas discharge laser system includes a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, and a resonator including the discharge chamber therein for generating a laser beam. The resonator further includes an adaptive optical element for making wavefront corrections. The adaptive optical element may include a plate with a non-planar refractive optical surface, a window on the discharge chamber with a non-planar refractive optical surface, an interferometric device with a non-planar optical surface, a non-dispersive, highly reflective mirror with a non-planar reflective optical surface, a partially reflective output coupling mirror with a non-planar reflective optical surface, or a polarizing plate with a non-planar refractive optical surface. The resonator may further include one or more line-narrowing optical elements for narrowing a bandwidth of the laser beam.
The non-planar surface of the adaptive optical element may have a contour fit to an average wavefront of the beam incident on the non-planar surface. The non-planar surface may have a contour fit to correct transient wavefront distortions. The non-planar surface of the adaptive optical element may have an adjustable contour, and the laser system may further include a detection and control system including a detector and a processor for monitoring a parameter of the laser beam indicative of a fitting of the contour of the non-planar surface of the adaptive optical element to a contour of the wavefront of the laser beam incident on the non-planar surface. The processor may be connected in a feedback loop with the adaptive optical element for controlling the contour of the non-planar surface of the adaptive optical element based upon information received from the detector. The detection and control system may further include a spectrometer.
In further view of the above, a gas discharge laser system is provided including a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a discharge circuit for energizing the gas mixture, and a resonator including the discharge chamber therein for generating a laser beam. The resonator further includes an adaptive optical element for making wavefront corrections. The adaptive optical element may include a plate with a non-planar refractive optical surface, a window on the discharge chamber with a non-planar refractive optical surface, an interferometric device with a non-planar optical surface, a transmission grating with a non-planar optical surface, a reflection diffraction grating with a non-planar reflective optical surface, a non-dispersive, highly reflective mirror with a non-planar reflective optical surface, a partially reflective output coupling mirror with a non-planar optical surface, or a polarizing plate with a non-planar refractive optical surface. The resonator may further include one or more line-narrowing optical elements for narrowing a bandwidth of the laser beam. The non-planar surface of the adaptive optical element has an adjustable contour, and the laser system further includes a detection and control system including a detector and a processor for monitoring a parameter of the laser beam indicative of a fitting of the contour of the adaptive optical element to a contour of a wavefront of the laser beam incident on the non-planar surface. The processor may be connected in a feedback loop with the adaptive optical element for controlling the contour of the non-planar surface of the adaptive optical element based upon information received from the detector. The detection and control system may further include a spectrometer.
In further view of the above, a method for making wavefront corrections to a laser beam generated by a gas discharge laser is provided. The laser is operated for generating the laser beam. The laser beam is reflected from an intracavity retro-reflective array for making wavefront corrections. The retro-reflective array may have a non-planar contour fit approximately to an average wavefront contour of the laser beam incident upon the array. The retro-reflective array may have an adjustable contour, wherein the contour is adjusted for approximately fitting a contour of a wavefront of the laser beam incident upon the array. The adjusting may be based on detection of a parameter of the laser beam indicative of a fitting of the contour of the retro-reflective array to the contour of the wavefront of the laser beam.
In further view of the above, a method for making wavefront corrections to a laser beam generated by a gas discharge laser is provided. The laser is operated for generating the laser beam. The laser beam is reflected from an intracavity phase conjugate mirror for making wavefront corrections.
In further view of the above, a method for making wavefront corrections to a laser beam generated by a gas discharge laser is provided. The laser is operated for generating the laser beam. The laser beam is reflected from an adaptive optical element for making wavefront corrections. The adaptive optical element may include an interferometric device with a non-planar optical surface, a non-dispersive, highly reflective mirror with a non-planar reflective optical surface, or a partially reflective output coupling mirror with a non-planar optical surface. A parameter of the laser beam indicative of a fitting of the contour of the non-planar surface of the adaptive optical element to a contour of a wavefront of the laser beam may be detected, and the contour of the non-planar surface of the adaptive optical element may be adjusted based upon information obtained in the detecting step for approximately fitting the contour of the wavefront of the laser beam incident upon the array for making wavefront corrections.
In further view of the above a method for making wavefront corrections to a laser beam generated by a gas discharge laser is provided. The laser is operated for generating the laser beam. The laser beam is transmitted through an adaptive optical element for making wavefront corrections. The adaptive optical element may include a plate with a non-planar refractive optical surface, a window on a discharge chamber of the laser with a non-planar refractive optical surface, an interferometric device with a non-planar optical surface, a transmission grating with a non-planar optical surface, or a polarizing plate with a non-planar refractive optical surface. A parameter of the laser beam indicative of a fitting of the contour of the non-planar surface of the adaptive optical element to a contour of a wavefront of the laser beam may be detected, and the contour of the non-planar surface of the adaptive optical element may be adjusted based upon information obtained in the detecting step for approximately fitting the contour of the wavefront of the laser beam incident upon the array for making wavefront corrections.